1. Field of the Invention
This invention relates generally to drilling of wellbores and more particularly to a fluid-handling system for use in underbalanced drilling of wellbores.
2. Background of the Art
In conventional drilling of wellbores for the production of hydrocarbons from subsurface formations, wellbores are drilled utilizing a rig. A fluid comprising water and suitable additive, usually referred to in the art as "mud," is injected under pressure through a tubing having a drill bit which is rotated to drill the wellbores. The pressure in the wellbore is maintained above the formation pressure to prevent blowouts. The mud is circulated from the bottom of the drill bit to the surface. The circulating fluid reaching the surface comprises the fluid pumped downhole and drill cuttings. Since the fluid pressure in the wellbore is greater than the formation pressure, it causes the mud to penetrate into or invade the formations surrounding the wellbore. Such mud invasion reduces permeability around the wellbore and reduces accuracy of measurements-while-drilling devices commonly used during drilling of the wellbores. Such wellbore damage (also known as the skin damage or effect) may extend from a few centimeters to several meters from the wellbore. The skin damage results in a decrease in hydrocarbon productivity.
To address the above-noted problems, some wells are now drilled wherein the pressure of the circulating fluid in the wellbore is maintained below the formation pressure. This is achieved by maintaining a back pressure at the wellhead. Since the wellbore pressure is less than the formation pressure, fluids from the formation (oil, gas and water) co-mingles with the circulating mud. Thus, the fluid reaching the surface contains four phases: cuttings (solids), water, oil and gas. Such drilling systems require more complex fluidhandling systems at the surface. The prior art systems typically discharge the returning fluids ("wellstream") into a pressure vessel or separator at the surface to separate sludge (solids), water, oil and gas. The pressure in the vessel typically exceeds 1000 psi. A number of manually controlled valves are utilized to maintain the desired pressure in the separator and to discharge the fluids from the pressure vessel. These prior art systems also utilize manually controlled emergency shut down valves to shut down the drilling operations. Additionally, these systems rely upon pressure measured at the wellhead to control the mud pressure downhole. In many cases this represents a great margin of error. These prior art fluid-handling systems require the use of high pressure vessels, which are (a) relatively expensive and less safe than low pressure vessels, (b) relatively inefficient, and (c) require several operators to control the fluid-handling system.
The present invention addresses the above-noted deficiencies of the prior art fluid-handling systems and provides a relatively low pressure fluid-handling system which utilizes remotely controlled fluid flow control devices and pressure control devices, along with other sensors to control the separation of the constituents of the wellstream. The present invention also provides means for controlling the wellbore pressure from the surface as a function of the downhole measured pressure.